The use of magnesium-containing compounds to catalyze the dimerization of hydroxypivaldehyde to hydroxypivalyl hydroxypivalate, by the so called Tischenko reaction, is well known in the art. For example, magnesium ethyl iodide has been used to catalyze this reaction, as described by Franke and Kohn in Monatsheft Fur Chemie, Vol. 25, page 865 (1904).
Representative of more recent work in this area is U.S. Pat. No. 3,862,215 (1975) wherein the use of magnesium hydroxide and magnesium oxide are described as catalyst for the conversion of hydroxypivaldehyde to produce hydroxypivalyl hydroxypivalate. The oxide or hydroxide catalysts disclosed in this reference are used in the presence or absence of water at concentrations ranging from 0.1 up to 20 wt %, based on the weight of the total reaction mixture, with concentrations preferably falling in the range of 1 up to 5 wt %. Catalyst is then removed from the crude product by filtration prior to further product purification by distillation.
The difficulties encountered in removing magnesium hydroxide or magnesium oxide catalysts from hydroxypivalyl hydroxypivalate reaction product by filtration of the reaction mixture are described by Merger and Deumbgen in U.S. Pat. No. 3,852,335 (1974). This reference teaches that filtration to remove reaction catalysts is greatly improved by neutralization of the reaction mixture with formic acid, thereby avoiding the formation of a pasty filter cake having low permeability.
An alternate means of dealing with the filtration problem is described in U.S. Pat. No. 4,665,219 (1987), wherein the removal of catalyst from the hydroxypivalyl hydroxypivalate-containing reaction mixture is accomplished in two steps. Catalyst is first neutralized, and then the resulting salts are removed by liquid-liquid extraction. The removal of basic calcium, barium, strontium, lithium, magnesium, cobalt, nickel, lead, bismuth, lanthanum, cerium, and zinc hydroxides, oxides and hydrated oxides to levels as low as 0.02% are suggested.
The presence of residual reaction catalysts and/or neutralization salts thereof, can cause difficulties in the further purification of the hydroxypivalyl hydroxypivalate product, because such metal-containing materials can cause decomposition and disproportionation of the desired product at the temperatures encountered in purification by distillation (typically in the range of about 130.degree. up to 200.degree. C.). The presence of such residual reaction catalysts and/or neutralization salts thereof can also foul such processing equipment as distillation column base heat exchangers or the blades of wiped film evaporators.
Other catalysts known in the art to be useful for the conversion of hydroxy aldehydes to produce dihydroxy esters include metal alkoxides of aluminum, sodium, and titanium. See, for example, Journal of the American Chemical Society, Vol. 74, pages 5133-5135 (1952).
Accordingly, improved methods for the production and recovery of dihydroxy esters by the catalytic reaction of hydroxy aldehydes would be desirable.